Slime by David Walliams. Ages 7+.
Slime (2020) is set on the Isle of Mulch — a strange isolated island where the adults are all villains. Ned is ten years old and bedridden with an illness. From his window he watches the misery the island's horrible adults inflict on everyone around them.
One day a blob of living slime appears at his window. It can take any shape. Together, Ned and Slime defeat the island's horrible adults one by one — each in a satisfyingly messy way. The novel is pure comic invention, funny and fast.